This problem is with a 89 model K5 with a 4 speed granny low trans. This truck is a friends machine and he has an issue with a popping noise while running in 4wheel drive. I rode with him and it sounded like it was coming right under the passenger side seat. It only does this while it is under a load no matter if it is cold or hot. He put a 4 inch lift on his K5 and this started about 2 weeks after that. He had a new drive shaft made for the front once this noise started. That didn't help so he got a GM man look inside the transfer case, nothing he could tell was a major problem, put new chain and gears in it while he was there, still no change. We have swapped hubs from one side to the other but the noise doesnt transfer, still under passenger side. We looked at both front and rear chunks and no shavings there to indicate a problem. Jacked it up and put it on jack stands so that tires would turn over but no noise untill it is under a load. Noise is not there when in 4 wheel drive withOUT hubs locked in. I think it has something to do with the pitch of the tranfercase, and needs to be angled down to make drive shaft more in line with front axle. I am open for things to try or look for, at this point the only thing I know to do is maybe replace hubs or shim tranfercase. At the end of my rope.

 

Did he get a different style driveshaft, one that can handle more of an angle than the original, or just a stock replacement.

Also, is the front driveshaft hitting the cross member?

 

the stock front driveshaft will handle a 4" lift. it should also clear the crossmember without any problems.
a few questions to help narrow down the problem ........
1-by under load do you mean in a straight line , while turning ,,etc...?
2-have you checked the front ball joints for play ?
3-have you checked the wheel bearings for proper adjustment ?
4-have you checked the steering knuckle u-joint ?

sounds to me like something on the passenger side isnt allowing the hub to fully lock and when put under a load its disengaging .